Force transducers for use in arrays

ABSTRACT

A force transducer is described which uses current pulses in a conductor to generate acoustic pulses in a magnetostrictive delay line. The pulses are sensed by a coil around the line. The magnitude of the pulses is modified by force applied to the transducer either by changing the shielding effect of a magnetoelastic ribbon (between the conductor and the delay line) by applying a force to be sensed thereto, or in another alternative by modifying the distance between the conductor and the delay line in accordance with applied force. The transducers are economic and easily assembled into arrays for sensing stress distribution in two dimensions because the common conductors can be used for the columns of the array and common delay lines can be used for the array rows. Each column then has a common current-pulse generator and each row has a common detector coil. A similar technique may be used for sensing the spatial distribution of other parameters such as temperature, magnetic field or displacement.

The present invention relates to force transducers particularly, but notexclusively, for use in stress arrays providing an indication of loaddistribution.

There are a large number of applications where it is useful to have anarray of transducers which can provide an indication of loaddistribution. These applications include walkways for use in clinicalpractice to obtain objective information on the gait characteristics ofpatients in order to evaluate degrees of disability and to assist in thediagnosis of locomotor disorders and assess response to treatment.Existing systems of this kind are either rather limited in the level ofdetail that is obtained or the array is limited to an area of the orderof 600 cm². An object of the present invention is to provide a moreprecise system which increases the sensitivity of diagnosis and has agreatly increased array area but is relatively inexpensive.

In order to make a large load sensitive it is necessary to provide ahigh number of transducers if a useful resolution is to be obtained.Thus another object of the invention is to provide economicaltransducers which are particularly suitable for use in such arrays.

There are many fields in which arrays of this type are useful. Forexample there are several other applications in the medical field:checking pressure distribution between an artificial limb socket and thelimb stump, monitoring pressure distribution on beds, seats andwheelchairs, in the study of pressure sores and the design ofcomfortable appliances. Outside the medical field applications includesports training, computer keyboards, touch sensitive electronic organs,monitoring long term stress in civil engineering structures and in soilmechanics, and tactile surfaces for robotic hands.

According to a first aspect of the present invention there is provided aforce transducer comprising

a magnetostrictive member which, in operation, acts as an acoustic delayline and is sensitive to changes in magnetic flux,

a conductor,

means for generating current pulses in the conductor, the conductorbeing positioned sufficiently close to the magnetostrictive member tocause an acoustic pulse to be launched in the magnetostrictive membereach time one of the said current pulses is generated,

a force receiving member,

modifier means for varying the amplitudes of the acoustic pulses independence upon force applied to the force receiving member, and

detector means for deriving electrical output signals having valuesdependent on the amplitudes of the acoustic pulses.

The modifier means may comprise a shielding member employingmagnetoelastic material having a magnetisation curve which variessignificantly with applied mechanical stress, the shielding member beingpositioned to modify the magnetic field in the region of themagnetostrictive member due to current pulses in the conductor as themechanical stress in the said material varies. The force receivingmember is then coupled to the shielding member to vary the mechanicalstress in the shielding member when the force applied to the transducervaries.

The magnetostrictive member may comprise a strip or an elongated wire ora bundle of strips or wires of magnetostrictive material such as nickel,the strip, wire or bundles usually, but not necessarily, being at rightangles to the conductor in order to give longitudinal magnetic flux inthe magnetostrictive member and so obtain maximum sensitivity. The wiremay be of any metal exhibiting the magnetostrictive effect but it ispreferably a drawn, or rolled, metal wire having a high permeability andhigh magnetostriction providing a delay line with a high and uniformsensitivity characteristic along its length.

The shielding member may be a ribbon of a metal alloy having highpermeability which is strongly affected by stress, for example theribbon may be of Metglas 2605SC or Vitrovac (Registered Trade Mark). Theribbon may run parallel to the conductor between the conductor and thedelay line. In operation the force applied to the transducer istransferred from the force receiving member to apply tensionlongitudinally in the ribbon.

The above mentioned metal alloys, Metglas and Vitrovac, may also be usedto form the magnetostrictive member.

The transducer may comprise two parallel conductors equally spaced fromthe delay line each with a sheet of magnetoelastic material between thedelay line and the conductor. In such an arrangement it is preferable toarrange that force applied to the transducer partially relieved anexisting tensile stress in one magnetoelastic sheet and increases anexisting tensile stress in the other, so that a differential effect isproduced. Thus the most non-linear parts of the response of themagnetoelastic material of the sheet may be avoided.

In an alternative form of the transducer of the first aspect of thepresent invention the magnetostrictive member may again comprise anelongated wire of magnetostrictive material, the conductor and the wirebeing at right angles but the force receiving member is, in thisalternative, arranged to modify the distance between the conductor andthe elongated wire in accordance with the force applied to thetransducer.

When the magnetostrictive member comprises a wire, the detector meansmay include a coil around the wire. The detector means may then providethe said output signals as signals each representative of an intervaldependent on the time a signal in the detector coil, resulting from acurrent pulse in the conductor, reaches a predetermined value as itincreases and/or decreases.

In an alternative arrangement which provides a binary output, theconductor partially loops the elongated wire and the force receivingmember is arranged to short-circuit the loop when a force greater than apredetermined threshold is applied to the transducer.

The above transducers are particularly useful in forming arrays of forcetransducers since common conductors can be used for the transducers inthe rows of the arrays and common delay lines can be used fortransducers forming columns of the array. In this case the delay linesare continuously sensitive to external magnetic field over the regioncoupled by the transducers. In addition the conductors can be connectedin groups to use common current pulse generating means and thetransducers in each column of the array may use common detector means inthe form of a coil around a wire forming the delay line of the column.Thus it is seen that transducer arrays can be constructed which areeconomical in comparison with arrays in which transducer outputs areindividually connected.

The array may be made without electrical joints in the area in whichstress is applied giving an important advantage over other arrays wheresuch joints are in the said area and often, therefore, repeatedlysubject to cyclic stress. The array can be manufactured by a simpleprocess without the need to have separate electrical joints at eachtransducer site.

According to a second aspect of the present invention there is provideda transducer comprising

an acoustic delay line which is sensitive to changes in magnetic flux,

a conductor,

means for causing current changes in the conductor, the conductor beingpositioned sufficiently close to the delay line for the changes toaffect acoustic waves in the delay line,

a receiving member,

modifier means for varying the amount by which the said changes affectthe said acoustic waves in dependence upon a condition at the receivingmember, and

detector means for deriving electrical output signals having valuesdependent on characteristics of acoustic waves in the delay line.

For example the said condition may be temperature magnetic field,mechanical stress or displacement.

According to a third aspect of the present invention there is provided atransducer array comprising a plurality of transducers each comprising

an acoustic delay line which is sensitive to changes in magnetic flux,

a conductor,

means for causing current changes in the conductor, the conductor beingpositioned sufficiently close to the delay line for the changes toaffect acoustic waves in the delay line,

a receiving member,

modifier means for varying the amount by which the said changes affectthe said acoustic waves in dependence upon a condition at the receivingmember, and

detector means for deriving electrical output signals having valuesdependent on charracteristics of acoustic waves in the delay line.

Certain embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing of a transducer according to theinvention,

FIG. 2 shows waveforms explaining how force magnitude can be obtainedfrom the transducer of FIG. 1,

FIG. 3 is a schematic drawing of an alternative transducer according tothe invention.

FIG. 4 is a magnetisation curve showing the effect of tension for anamorphous metallic alloy,

FIG. 5 is a schematic drawing of another transducer according to theinvention,

FIG. 6 is a schematic drawing showing another view of the transducer ofFIG. 5,

FIG. 7 is a schematic drawing of an array of transducers according tothe invention.

FIG. 8 is a drawing showing how the transducers of the type shown inFIG. 1 can be used in the array of FIG. 7,

FIG. 9 shows a modified form of transducer of FIG. 1 according to theinvention for use in the array of FIG. 7,

FIG. 10 is a schematic drawing of part of the wiring of one form of anarray employing transducers according to the invention, and

FIG. 11 is a block diagram of a computer controlled system for operatingthe array of FIG. 10.

FIG. 1 shows a schematic example of a transducer employing amagnetostrictive delay line 10 consisting of a nickel wire or a strip ofan amorphous magnetic alloy such as Metglas 2605SC. A pulse generator 11generates a series of pulses which are passed by way of an an amplifier12 to a conductor 13 in the region of the delay line and at right anglesthereto. Each time a current pulse appears in the conductor 13 theresulting pulsed magnetic field generates a local stress in the delayline 10 due to the high magnetostriction constant of nickel or theamorphous magnetic alloy. Consequently an acoustic pulse passes alongthe wire at about 5 km/s. A detector coil 14 is wound round the wire 10and receives a bias flux from a bar magnet 9, or equivalent. When theacoustic pulse arrives at the receiving coil 14 a voltage pulse isgenerated in the coil 14 due to the change in the magnetic flux appliedto the coil arising from modulation of the permeability of the delayline. These voltage pulses are applied to a wide-band amplifier 15 andthen to a comparator 16 which receives a threshold voltage as shown.

A smooth channel is provided to position the delay line in relation toother components of the transducer but is not shown in FIG. 1; forexample a nylon tube may be used. Such a channel is preferably providedfor the other transducers described in this specification and defined inthe claims.

In operation as a force transducer, a distance 17 between the wire 13and the delay line 10 is varied according to the force to be measuredand as a result the amplitude of pulses received at the output of theamplifier 15 varies, since the size of the acoustic pulses in the delayline 10 depends on the distance 17. The comparator 16 is then used toprovide an output only when pulses generated in the coil 14 are greaterthan a certain amplitude providing an indication of whether or not aforce has been applied to decrease the distance 17.

In general, means are preferably required to establish a bias field atthe point where the conductor crosses the delay line. This may forexample be by means of a small permanent magnet 6 but in some cases canbe achieved by premagnetising the delay line. These remarks apply to allthe transducers described in this specification and defined in theclaims.

Instead of moving the conductor in relation to the delay line thepermanent magnet 6 which provides a static bias field can be moved, forexample, as indicated by the arrow 5, in response to applied force andthis alters the amplitude of the acoustic pulses generated.

The main application of such transducers is in economical arrays whereeach crossing of a delay line such as the delay line 10 by a conductorsuch as the conductor 13 corresponds to a point in the array. Thesearrays will be described in more detail later.

As described, the transducer of FIG. 1 is a binary device in that itindicates only whether a force has been applied or not. The transducercan be modified to indicate the magnitude of the force applied byreplacing the comparator 16 with an analogue-to-digital converter for adigital output or by an analogue indicating device which indicates thepeak magnitudes of the pulses from the amplifier 15.

Alternatively force magnitude may be obtained by measuring the time forwhich each pulse from the amplifier 15 remains above the threshold. FIG.2 shows two amplifier pulses 15' which result from pulses 11' from thegenerator 11. The upper waveforms show a relatively large pulse 15'resulting from a force which considerably reduces the distance 17 andthe lower waveforms show a relatively small pulse due to a force whichreduces the distance by a small amount. The times T₁ and T₂ for whichthe two pulses are above the threshold shown, provide an indication ofthe applied forces, and the threshold can be arranged so that when noforce is applied the threshold is not crossed.

The output of the comparator 16 may be coupled as one input to atwo-input AND gate (not shown) while a further pulse generator (notshown) is coupled to the other AND gate input. The output of the ANDgate is coupled to a counter (not shown) so that if the pulses from thefurther generator are generated at a much higher rate than those fromthe generator 11, and the counter is reset by each pulse 11', then themaximum count reached by the counter is representative of applied force.

Force magnitude may be obtained in another way by measuring the timebetween the application of each pulse 11' by the generator 11 and thetime the leading or trailing edge of a pulse 15' from the amplifier 15crosses the threshold voltage of the comparator 16. However, the time tothe leading edge is inversely related to the force applied to thetransducer.

An alternative structure for the conductor 13 in a binary transducer isa loop which wholly or partially encloses the delay line 10 but which isshort-circuited when a force is either applied to, or removed from, thetransducer. Such a transducer is shown in FIG. 9.

Another transducer is shown in FIG. 3 where, as is the case throughoutthis specification, those parts having the same functions have the samedesignations as in FIG. 1. In FIG. 3 a ribbon 18 of amorphous metalalloy ribbon such as Metglas having strong magnetoelasticcharacteristics is positioned between the conductor 13 and the delayline 10. The magnetisation characteristics of the ribbon material showsthat permeability is very sensitive to mechanical stress (see FIG. 4which is from "Magnetic Sensors of New Materials" by Boll and Borek,Siemens Forsch.--u. Entwickl.--Ber. Bd. 10 (1981) Nr. 2). In theunstressed condition the permeability is high and the ribbon acts as amagnetic screen between the conductor 13 and the delay line 10 but whenthe ribbon is stressed the permeability is greatly reduced and more fluxpenetrates the delay line 10 when the current pulse occurs. Thus byapplying tension as indicated by the arrow 19 the amplitude of voltagepulses generated at the coil 14 is increased. The ribbon 18 is attachedto a support 7 and spaced from the delay line 10 by a spacer 8 since thedistance between the ribbon and the delay line must be kept constant.

In a typical example the delay line 10 is spaced by about 0.25 mm fromthe strip 18 and the wire 13 is an insulated copper conductor withinsulation touching the strip 18. The detector coil 14 has 1,000 turnsand the voltage amplification provided by the amplifier 15 is about 200.In these circumstances the peak amplitude of the pulses at the output ofthe amplifier 15 was approximately proportional to the stress andbetween 1 and 10 volts for rectangular current pulses in the conductor13 of about 20 amps and of about 3 microsec duration. The maximum stressin the ribbon 18 was calculated very approximately as 70 Newtons/sq mmin the series of measurements giving the above values.

It may be preferable to arrange for the flux in the ribbon 18 to be inthe same direction as the tension, and for this reason the ribbon 18 maybe positioned parallel to the delay line 10 with tension appliedparallel to the axis of the delay line.

An improved transducer employing magnetoelastic strips is shown in FIGS.5 and 6. The pulsed conductor 13 is replaced by two parallel conductors20 and 21 equally spaced from the delay line 10, and shielded bymagnetoelastic sheets 22 and 23, respectively.

Magnetic fields due to equal currents in the conductors 20 and 21 areequal and opposite at the delay line 10 if the shielding effects of thesheets 22 and 23 are equal. Hence the net effect of the pulsed currentsis zero, that is a zero amplitude acoustic pulse is generated, and thusa zero-amplitude voltage pulse appears at the expected time. However ifa force to be measured is applied in the direction of the dashed arrow24 in such a way that an existing tensile stress in the upper sheet 22is partially relieved and an existing tensile stress in the lower sheet23 is increased, then the shielding effect of the upper sheet becomesgreater and that of the lower sheet becomes smaller with the result thatthe acoustic pulses generated in the delay line 10 have significantamplitude of positive or negative sign. Since this is a differentialarrangement many spurious common mode effects are much reduced.

FIG. 6 shows one way in which a reduction and an increase in tension inthe sheets 22 and 23 can be achieved by the application of a force. Thesheets 22 and 23 are intimately fixed to the outer surfaces of a squaresection tube 25 which is a non-magnetic tube, made for example ofaluminium, brass, copper or a ceramic. The tube encloses the delay line10 but the conductors 20 and 21 are outside. Supports 26 and 27 supportthe ends of the tube and force is applied in the direction shown by thearrow 24 by way of a bridge member 28 which bridges the sheet 22 andapplies the load over a fairly wide area. The tube 25 is prestressed (bymeans not shown) so that both its upper and lower surfaces in the regionof the sheets 22 and 23 are in tension, for example, by means of ahollow longitudinal bolt inside the tube 25 and arranged by means ofsuitable terminations to apply tension to the tube. Prestressing may beachieved by applying longitudinal tension to the tube. The applicationof the force in the direction of the arrow 24 then causes a decrease inthe tensile stress in the upper surface of the tube 25 and an increasein the tensile stress in the lower surface.

Other types of differential transducers according to the inventioninclude transducers similar to those of FIG. 5 but without the sheets 22and 23 and with an additional member comprising magnetic material, suchas Mumetal or ferrite. The additional member, which may be disc shaped,is positioned above the conductor 20 (as seen in FIG. 5) and when aforce is applied to the transducer moves towards this conductorupsetting the balanced arrangement and generating acoustic pulses in thedelay line. Alternatively two such additional members may be used whichnominally form a symmetrical structure and the transducer input causesmovement of one or both additional members.

Alternatively transducers similar to those of FIG. 5 but without thesheets 22 and 23 may simply allow the positions of the conductors 20 and21 to vary in relation to the delay line 10 on the application of aforce. The conductors may then be supported by resilient layers whileprotection from stress is provided for the delay line.

As has been mentioned force transducers for example of the types shownin FIGS. 1, 3, 5 and 6 and related transducers can be usefully arrangedin arrays to allow the distribution of forces over two dimensional areasto be measured and/or compared.

An example of such an array is shown in FIG. 7 where a number of nickelwire delay lines, one of which is designated 10, are positioned parallelto one another, in respective grooves in a base member 30, and separatedby a distance of, for example, 10 mm. Each of these wires has adetection coil, one of which is designated 14, connected to an amplifier15. The magnets 9 are also present but, for clarity of the drawing, arenot shown in FIG. 7. Parallel conductors are positioned at right anglesto the delay lines and separated from them by a compressible layer 31.The whole arrangement is covered by a flexible layer 32.

In this arrangement the transducers are of the type shown in FIG. 1since when a pressure distribution is applied to the top of the layer 32the compressible layer 31 compresses by different amounts according tothe pressure distribution reducing the distance between the conductorsand the delay lines according to the distribution. Since the delay linesare positioned in grooves, they are protected from the applied pressuredistribution. This is importance since pressure would change theacoustic properties of the delay lines.

In order to localise each transducer so that its output is independentof forces applied to neighbouring transducers the construction at eachcross-over may be as shown in FIG. 8 where the conductors 13 aresupported by islands of compressible material, one for each cross-over,and each conductor is provided with partial loops 33 where it crossesgrooves between islands. The delay lines 10 are located in protectivenylon tubes 10'.

One way of constructing transducers, mentioned above, is to arrange forthe conductors to loop partially round the delay lines. The partialloops are then short-circuited on the application of pressure. Such anarrangement is outlined in FIG. 9 where a circular conductor 35 isnormally spaced from the conductor 13. When a force is applied to thelayer 32 in the region of the circular conductor 35, this conductorcontacts the ends of the partial loop and short-circuits the partialloop so that the amplitude of acoustic pulses in the delay line 10 isgreatly reduced. The conductor 35 and similar conductors for the othertransducers in the array may, for example, be etched by circuit boardmanufacturing techniques from a conductive layer on a backing insulatingsheet.

In practice it is found that each acoustic pulse occupies about 5 cm ofthe wire 10 so that in order to obtain discrete output pulses from theamplifier 15 those conductors which are simultaneously excited must beseparated by at least 5 cm, 8 cm being chosen in practice. However inorder to give the same resolution in both dimensions of the array,parallel conductors must be provided which are the same distance apartas the delay lines, that is 10 mm. This is achieved by connecting theconductors in interleaved groups as indicated in FIG. 10. Beforedescribing FIG. 10 in more detail, one other phenomenon will bementioned. On reaching the ends of the delay lines the acoustic pulsesare reflected and thus a single pulse in one of the parallel conductorstends to cause a reverberation which may last for many transits alongthe delay line 10. To interrogate repetitively it is necessary to waituntil this activity has died away before generating another pulse in thedelay line. Thus an interval is required before more pulses may begenerated in the parallel conductors. In order to achieve maximuminterrogation rate, damping structures are required to reduce reflectionat the ends of the delay lines 10 to a very low level. Nearly any formof coating at the ends of the conductors can be used for damping but, inparticular to avoid further reflections, the coating must be appliedwithout forming an abrupt discontinuity. Materials which may be usedinclude latex or a deposited tapered metal layer such as a lead layer.In practice reflections can be made insignificant within about threetransit times, that is about 33 mS for delay lines 5 m in length, thusallowing a maximum sampling rate of 330 interrogations per second.

Eight pulse generators are used, by way of example, in the array of FIG.10, although a single pulse generator and a de-multiplexer is analternative. Only the first three of these generators (37, 38 and 39)and the last (the generator 40) are shown. Each pulse generator isconnected to a number of the parallel conductors, for example thegenerator 37 is connected to the conductors 42 to 45, and theseconductors are spaced by 8 cm. The next pulse generator 38 is connectedto a group of conductors 46 to 49 which are interleaved with theconductors connected to the generator 38, corresponding conductors inthe two groups being separated by one centimeter. Conductors in theother groups are similarly spaced from those of neighbouring groups andin this way eight groups of conductors are formed, each with itsconductors connected to one of the pulse generators.

In operation the pulse generators are triggered in turn, the intervalbetween triggering being 3 mS but each pulse generator producing nsamples each 3 mS where n is the number of conductors in each of thegroups.

The delay lines can conveniently be connected in groups of eight, suchas the groups 51, 52 and 53, and where the transducers are, for example,of the binary form shown in FIG. 9, each group of eight delay lines canbe regarded as giving rise to a single byte of data each time one of theconductors crossing that group of delay lines is pulsed. Thus forexample a single pulse from the generator 37 generates four successivebytes from the group of delay lines 51, one from each of the conductors42 to 45, and four successive bytes from each of the other groups ofdelay lines 52, 53 and so on.

An array 57 such as that of FIG. 10 may be controlled by a computer 55as shown in FIG. 11. The computer initiates an interrogation cycle on aselected pulse generator in a group 56, which comprises the pulsegenerators of FIG. 10 by sending out a channel number. This numberselects one of the pulse generators and a group of conductors in thearray 57 is pulsed. As a result a series of acoustic pulses appears oneach one of the delay lines of the array and passes to a detector,amplifier and comparator, for example as shown in FIG. 1. These outputsare grouped into sets of eight to form data bytes and applied to amultiplexer 59. Since the acoustic pulses are of comparatively longduration, the bytes in the multiplexer can be read serially along aneight bit bus 61 into a buffer memory 62 while the outputs from thecomparators remain at steady values. Addressing the buffer memory 62 isunder the control of logic 63 which receives an eight bit input from acounter 64. Each time a pulse generator provides an output, theaddressing logic serially addresses each byte location in a group ofbyte locations determined by the address received from the counter 64.The data bytes from the multiplexer 59 are then read serially atrelatively high speed into these locations. The counter 64 isincremented by pulses from a reference delay line so that successivepulses in the series of pulses are allocated so further subdivisions ofmemory.

When each pulse generator has been triggered the buffer memory 62 holdsa complete output from the array giving binary data for each transducer.The buffer memory can then be read by way of a bus 65 into a screenmemory held by the computer and controlling a display. Thus the displayis a binary interpretation of the load distribution on the array and thedata can be used for other computational purposes.

Where it is required to obtain an indication of the magnitude of thepressure applied at each transducer in the array then the comparators ofFIG. 1 and in the group 58 of FIG. 11 are replaced byanalogue-to-digital converters whose output is read into the multiplexer59. Alternatively the "delay" method of obtaining force magnitudedescribed in connection with FIG. 2 may be used. Each connection to themultiplexer (corresponding to one of the delay lines) now has a numberof data bits representing pressure, for example one byte. Thus themultiplexer, the addressing logic and the counter need to be modifiedaccordingly.

It may be necessary in some cases in order to give more time for datainput to the computer, for the detector coils on the delay lines to beat differing distances from the edge of the array so that the series ofpulses which are produced from each amplifier 15 falls within an overallseries of pulses distributed in a read-out cycle initiated by a pulsefrom one pulse generator.

It will be evident that the invention can be put into practice in manyother ways than those specifically described above. In particular,current changes in other forms than rectangular pulses may be used inthe conductors of the transducers and the transducers may respond toother input conditions than force or change in force.

I claim:
 1. A force transducer comprisinga magnetostrictive memberwhich, in operation, acts as an acoustic delay line and is sensitive tochanges in magnetic flux, a conductor, means for generating currentpulses in the conductor, the conductor being positioned sufficientlyclose to the magnetostrictive member to cause an acoustic pulse to belaunched in the magnetostrictive member each time one of the saidcurrent pulses is generated, a force receiving member, modifier meansfor varying the amplitudes of the acoustic pulses in dependence uponforce applied to the force receiving member, and detector means forderiving electrical output signals having values dependent on theamplitudes of the acoustic pulses.
 2. A transducer according to claim 1wherein the modifier means comprisesa shielding member employingmaterial having a magnetization curve which varies significantly withapplied mechanical stress, the shielding member being positioned tomodify the magnetic field in the region of the magnetostrictive memberdue to current pulses in the conductor as the mechanical stress in thesaid material varies, and the force receiving member being coupled tothe shielding member to vary the said mechanical stress when forceapplied to the force receiving member varies.
 3. A transducer accordingto claim 2 whereinthe magnetostrictive comprises at least one elongatedstrip or wire of magnetostrictive material, the conductor and the stripor wire are at right angles to one another, and the shielding membercomprises a sheet of the said material interposed between the conductorand the strip or wire.
 4. A transducer according to claim 3 whereinthesaid sheet is an elongated ribbon, and the force receiving member, inoperation, tensions the ribbon longitudinally.
 5. A transducer accordingto claim 3 comprisinga further conductor parallel to the otherconductor, and a further shielding member comprising a sheet of the saidmaterial interposed between the further conductor and the said elongatedstrip or wire, the two conductors being on either side of the saidelongated wire at equal distances therefrom, the two shielding membersbeing on either side of the said elongated wire at equal distancestherefrom, and the modifier means being arranged to relieve, partially,an existing tensile stress in one shielding member and to increase anexisting tensile stress in the other shielding member when force isapplied to the force receiving member.
 6. A transducer according toclaim 2 wherein the said material is chosen from the group comprising:ametal alloy having high permeability which is strongly affected bystress, Metglas 2605SC, and Vitrovac (Registered Trade Mark).
 7. Atransducer according to claim 1 whereinthe modifier means is arranged tovary the distance between the conductor and the magnetostrictive memberin response to force applied to the force receiving member.
 8. Atransducer according to claim 7 whereinthe magnetostrictive membercomprises at least one elongated strip or wire of magnetostrictivematerial, the conductor and the strip or wire are at right angles to oneanother, and the modifier means comprises resilient means between theconductor and the strip or wire.
 9. A transducer according to claim 1comprisinga further conductor parallel to the said conductor, andwherein the magnetostrictive member comprises at least one elongatedstrip or wire of magnetostrictive material, the two conductors are oneither side of the said elongated wire or strip and, in a datumposition, at equal distances therefrom, and the modifier means isarranged to increase the distance between one of the conductors and thestrip or wire and to decrease the distance between the other of theconductors and the strip or wire in response to forces applied to theforce receiving member.
 10. A transducer according to claim 1comprisinga further conductor parallel to the said conductor, andwherein the magnetostrictive member comprises at least one elongatedstrip or wire of magnetostrictive material, the two conductors are oneither side of the said elongated wire or strip at equal distancestherefrom, the modifier means comprises magnetic material which ispositioned on that side of one of the said conductors which is remotefrom the other conductor, and the force receiving member is arranged tovary the distance between the magnetic material and the said oneconductor in response to forces applied to the force receiving member.11. A transducer according to claim 10 whereinthe modifier meanscomprises further magnetic material which, in a datum position, forms asymmetrical arrangement with the other said magnetic material withrespect to the magnetostrictive member and the conductors.
 12. Atransducer according to claim 1 whereinthe magnetostrictive membercomprises at least one elongated strip or wire of magnetostrictivematerial, the conductor and the strip or wire are at right angles to oneanother with the conductor forming at least a partial loop around thestrip or wire, and the modifier means includes a conductive memberarranged to short-circuit the said loop in response to force applied tothe force receiving member.
 13. A transducer according to claim 1whereinthe magnetostrictive member comprises at least one elongatedstrip or wire of magnetostrictive material, the conductor and the stripor wire are at right angles to one another, and the detector meanscomprises a detector coil around the strip or wire and means forapplying a magnetic flux longitudinally of the strip or wire in theregion surrounded by the coil.
 14. A transducer according to claim 1wherein the detector means comprisesmeans for deriving electrical pulsesrepresentative of respective acoustic pulses, and means for obtainingsignals each representative of an interval dependent on the time atwhich one of the said electrical pulses reaches a predetermined leveleither as its level increases and/or decreases.
 15. A transduceraccording to claim 14 wherein the means for obtaining signalsrepresentative of the said intervals comprises means for obtaining anindication of the interval for which each of the said pulsesrepresentative of an acoustic pulse has a magnitude greater than athreshold magnitude.
 16. A transducer according to claim 1 whereinthemodifier means comprises a magnet in the vicinity of themagnetostrictive member, and the force receiving member is arranged tovary the distance between the magnet and the magnetostrictive member inresponse to applied force.
 17. A transducer according to claim 1including means for applying a magnetic bias field to themagnetostrictive member where the conductor is adjacent thereto.
 18. Aforce transducer array comprising a plurality of force transducers eachcomprisinga magnetostrictive member which, in operation, acts as anacoustic delay line and is sensitive to changes in magnetic flux, aconductor, means for generating current pulses in the conductor, theconductor being positioned sufficiently close to the magnetostrictivemember to cause an acoustic pulse to be launched in the magnetostrictivemember each time one of the said current pulses is generated, a forcereceiving member, modifier means for varying the amplitudes of theacoustic pulses in dependence upon force applied to the force receivingmember, and detector means for deriving electrical output signals havingvalues dependent on the amplitudes of the acoustic pulses.
 19. Atransducer array according to claim 18 whereinthe array is a planararray, the force transducers in the array are arranged in rows andcolumns, the magnetostrictive members of the transducers in each rowcomprise at least one common elongated strip or wire of magnetostrictivematerial particular to that row, the conductors of the transducers ineach column comprise a single common conductor, particular to thatcolumn, and the said strips or wires are parallel to one another and atright angles to the said conductors which are also parallel to oneanother.
 20. A transducer array according to claim 19 whereinthemodifier means of each transducer comprises a first common resilientlayer between the conductors and the strips or wires, the forcereceiving means of each transducer comprises a second common resilientlayer which when deformed in a region approximately equal to the area ofone of the transducers, deforms the first common resilient layer over anapproximately equal corresponding area, and means are provided forprotecting the strips or wires from stress due to forces applied to theforce receiving means.
 21. A transducer array according to claim 19whereinthe force receiving meas of each transducer comprises a commonresilient layer, and wherein in each transducer the conductor forms atleast a partial loop round the wire, and the modifier means includes aconductive member arranged to short-circuit the said loop in response toforce applied to the common resilient layer in the vicinity of thattransducer.
 22. A transducer array according to claim 19 whereinthecommon conductors are equally spaced, the common conductors are arrangedin groups with the distance between the conductors in each group greaterthan the distance occupied by an acoustic pulse in each of the commonstrips or wires, the groups of conductors are arranged in a series withthe distance between the conductors of each group and those of the nextgroup in the series being the distance between the conductors of eachgroup divided by the number of groups.
 23. A transducer array accordingto claim 21 whereinthe means for generating current pulses in eachtransducer comprises a common pulse generator for all the transducershaving common conductors connected in one said group, the common pulsegenerator being connected to the conductors of those transducers, andthe detector means in each transducer comprises a common detector coilfor all the transducers having a common said magnetostrictive wire, thecommon detector coil being located around the said wire of thosetransducers.
 24. A transducer array according to claim 18 wherein eachtranducer includes means for applying a magnetic bias field to themagnetostrictive member where the conductor is adjacent thereto.